Rotary disk filters of the type mentioned above are usually employed for filtering water in wastewater treatment plants. A usual design of a rotary disk filter according to prior art is schematically shown in FIG. 9.
An example of such a rotary disk filter is disclosed in SE-C-224,131. In this filter, water is conducted through one end of a central rotatable drum and via openings in the circumference of the drum radially outwards to disk-shaped filter chambers. Each of the filter chambers is defined by a disk-shaped filter member having opposing filter portions which are supported by an annular filter support unit arranged between the same. The filter members are mounted in parallel along the longitudinal axis of the drum. When water flows out through the filter portions, particles are retained in the filter chambers. When cleaning the filter portions, the drum is rotated and water is flushed onto the filter portions from outside in the upper area of the rotary disk filter, particles and water flowing into the upper area of the drum and being collected in a trough extending through the drum. The filter portions comprise annular filter cloth portions arranged on the sides of the filter support units.
SE-B-465,857 discloses a rotary disk filter of a similar kind, in which the disk-shaped filter members comprise a plurality of separate, disk-shaped filter sections, which together establish annular filter members. By the annular filter members being divided into a plurality of separate units, also the filter cloth is divided into smaller pieces, which means that in case of a local damage to the cloth a replacement of the cloth is necessary on only one of the filter sections, not on an entire annular disk.
In the two rotary disk filters described above, the filter cloth can be fastened in one of a plurality of ways. In a common solution, the filter cloth is glued directly to the filter support unit on the opposing sides thereof. This is particularly common when the cloth consists of some textile or plastic material. The cloth can Also be made of metal. In that case, it is often welded to the filter support unit, and if necessary, reinforcement ribs are welded to the outside of the filter cloth for improved securing thereof. In a further way of fixing the cloth to the support unit, the cloth is designed as a "bag", which is slipped around a filter support unit and is shrunk on the same.
Rotary disk filter constructions of this kind suffers from several problems. The filter cloth has a limited life in normal use and must be replaced at regular intervals. Moreover, the filter cloth is sensitive and can easily be damaged, necessitating a premature replacement thereof. If the cloth is damaged, an entire filter cloth portion must be replaced.
In all the ways described above, it is an extremely difficult and complicated operation to remove an old cloth and fasten a new cloth on the filter support unit, which means that this operation must be carried out by qualified experts and in a convenient industrial environment. Therefore the work is usually carried out by manufacturers of rotary disk filters, which means that entire rotary disk filter support units, which are annular or in the shape of annular sections, must be transported, in many cases over long distances, to the place of repair, even if the filter cloth has been damaged only locally on one side of the filter support unit. This causes a number of problems.
It is a heavy and difficult operation to dismount filter support units in a rotary disk filter, which in itself results in long down-times. At the same time there is a risk that, in connection with this operation, the filter cloth of neighbouring filter members is damaged unintentionally. Moreover, several persons are required for the dismounting and mounting work.
Transporting the filter support units for replacement of filter cloth is per se an expensive operation. This also takes time, and therefore a number of replacement filter support units provided with new filter cloth must be kept in stock, thereby avoiding long down-times.
Cloth that has been glued or welded to the support unit has a tendency to be split away from the filter support unit in operation, which per se leads to a premature exchange of the filter cloth being necessary.
Another problem of today's rotary disk filter designs is that when cutting filter cloth for disk-shaped or disk section-shaped filter support units, a large amount of filter cloth will be wasted. This draw-back is particularly pronounced when using so-called microfilter cloth which has very small filter apertures and is expensive to manufacture.
EP-B-0 684 868 discloses a different type of rotary disk filter which is intended for filtration from the outside of a filter member as liquid flows in from outside. Such a filter suffers from a number of draw-backs. For instance, it is practically impossible to clean the filter cloth by flushing in the direction opposite to the filtering direction. In contrast to this construction, the invention concerns a rotary disk filter having an opposite direction of flow, thus requiring completely different constructional details.